In the commercial production of crops, it is desirable to easily and quickly eliminate unwanted plants (i.e., “weeds”) from a field of crop plants. An ideal treatment would be one which could be applied to an entire field but which would eliminate only the unwanted plants while leaving the crop plants unharmed. One such treatment system would involve the use of crop plants which are tolerant to a herbicide so that when the herbicide was sprayed on a field of herbicide-tolerant crop plants, the crop plants would continue to thrive while non-herbicide-tolerant weeds were killed or severely damaged. Ideally, such treatment systems would take advantage of varying herbicide properties so that weed control could provide the best possible combination of flexibility and economy. For example, individual herbicides have different longevities in the field, and some herbicides persist and are effective for a relatively long time after they are applied to a field while other herbicides are quickly broken down into other and/or non-active compounds. An ideal treatment system would allow the use of different herbicides so that growers could tailor the choice of herbicides for a particular situation.
Crop tolerance to specific herbicides can be conferred by engineering genes into crops which encode appropriate herbicide metabolizing enzymes and/or insensitive herbicide targets. In some cases these enzymes, and the nucleic acids that encode them, originate in a plant. In other cases, they are derived from other organisms, such as microbes. See, e.g., Padgette et al. (1996) “New weed control opportunities: Development of soybeans with a Roundup Ready® gene” and Vasil (1996) “Phosphinothricin-resistant crops,” both in Herbicide-Resistant Crops, ed. Duke (CRC Press, Boca Raton, Fla.) pp. 54-84 and pp. 85-91. Indeed, transgenic plants have been engineered to express a variety of herbicide tolerance genes from a variety of organisms, including a gene encoding a chimeric protein of rat cytochrome P4507A1 and yeast NADPH-cytochrome P450 oxidoreductase (Shiota et al. (1994) Plant Physiol. 106: 17). Other genes that confer tolerance to herbicides include acetohydroxy acid synthase (“AHAS”), mutations in the native sequence have been found to confer resistance to multiple types of herbicides on plants expressing it and has been introduced into a variety of plants (see, e.g., Hattori et al. (1995) Mol. Gen. Genet. 246: 419); glutathione reductase and superoxide dismutase (Aono et al. (1995) Plant Cell Physiol. 36: 1687); and genes for various phosphotransferases (Datta et al. (1992) Plant Mol. Biol. 20: 619).
One herbicide which has been studied extensively is N-phosphonomethylglycine, commonly referred to as glyphosate. Glyphosate is a broad spectrum herbicide that kills both broadleaf and grass-type plants due to inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (also referred to as “EPSP synthase” or “EPSPS”), an enzyme which is part of the biosynthetic pathway for the production of aromatic amino acids, hormones, and vitamins. Glyphosate-resistant transgenic plants have been produced which exhibit a commercially viable level of glyphosate resistance due to the introduction of a modified Agrobacterium CP4 EPSPS. This modified enzyme is targeted to the chloroplast where, even in the presence of glyphosate, it continues to synthesize EPSP from phosphoenolpyruvic acid (“PEP”) and shikimate-3-phosphate. CP4 glyphosate-resistant soybean transgenic plants are presently in commercial use (e.g., as sold by Monsanto under the name “Roundup Ready®”).
Other herbicides of interest for commercial crop production include glufosinate (phosphinothricin) and acetolactate synthase (ALS) chemistry such as the sulfonylurea herbicides. Glufosinate is a broad spectrum herbicide which acts on the chloroplast glutamate synthase enzyme. Glufosinate-tolerant transgenic plants have been produced which carry the bar gene from Streptomyces hygroscopicus. The enzyme encoded by the bar gene has N-acetylation activity and modifies and detoxifies glufosinate. Glufosinate-tolerant plants are presently in commercial use (e.g., as sold by Bayer under the name “Liberty Link®”). Sulfonylurea herbicides inhibit growth of higher plants by blocking acetolactate synthase (ALS). Plants containing particular mutations in ALS are tolerant to the ALS herbicides including sulfonylureas. Thus, for example, sulfonylurea herbicides such as Synchrony (a mixture of chlorimuron-ethyl plus thifensulfuron-methyl) can be used in conjunction with ALS herbicide-tolerant plants such as the STS® soybean (Synchrony tolerant soybean) variety which contains a trait that enhances the soybean's natural tolerance to soybean sulfonylurea herbicides.
While a number of herbicide-tolerant crop plants are presently commercially available, one issue that has arisen for many commercial herbicides and herbicide/crop combinations is that individual herbicides typically have incomplete spectrum of activity against common weed species. For most individual herbicides which have been in use for some time, populations of herbicide resistant weed species and biotypes have become more prevalent (see, e.g., Tranel and Wright (2002) Weed Science 50: 700-712; Owen and Zelaya (2005) Pest Manag Sci. 61: 301-311). Transgenic plants which are resistant to more than one herbicide have been described (see, e.g., WO2005/012515). However, improvements in every aspect of crop production, weed control options, extension of residual weed control, and improvement in crop yield are continuously in demand.
Particularly, due to local and regional variation in dominant weed species as well as preferred crop species, a continuing need exists for customized systems of crop protection and weed management which can be adapted to the needs of a particular region, geography, and/or locality. For example, a continuing need exists for methods of crop protection and weed management which can reduce: the number of herbicide applications necessary to control weeds in a field; the amount of herbicide necessary to control weeds in a field; the amount of tilling necessary to produce a crop; and/or programs which delay or prevent the development and/or appearance of herbicide-resistant weeds. A continuing need exists for methods of crop protection and weed management which allow the targeted use of particular herbicide combinations.